In humans and other animals, strenuous exercise as well as exposure to sunlight and heat can result in significant physiological changes. Subjects exercising or working in the heat are at risk for developing heat related injuries. Environmental heat illnesses include heat syncope, heat exhaustion, dehydration syndrome, and heat stroke. The potentially fatal clinical syndrome of heat stroke has been described in marathon runners, military recruits, football players, and in hot industrial environments. An epidemic appearance of heat stroke has been described during heat waves in urban areas (Ferguson, M., and M. M. O'Brien [1960] "Heat Stroke in New York City: Experience with 25 Cases," N.Y. State J. Med. 60:2531-2538).
The "dehydration syndrome" is characterized by loss of appetite and limited capacity for work. Evidence of heat exhaustion becomes apparent with losses of 5% of the body water, and at 7% disorientation and hallucinations occur. Losses of body water of 10% or greater are extremely hazardous and lead to heat stroke and death if not treated immediately. Heat stroke is accompanied by high body temperature (106.degree.-110.degree. F.), deep coma, and in most cases there is complete absence of sweating, and failure of the major organ systems.
Three factors determine the thermal balance of the body: metabolic heat production, heat exchange between the organism and its surroundings, and heat loss by the evaporation of sweat (Knochel, J. P. [1980] "Clinical physiology of heat exposure," In Clinical Disorders of Fluid and Electrolyte Metabolism, M. H. Maxwell and C. R. Kleeman, eds., McGraw-Hill, New York). For the subject exercising or working, particularly in a hot environment, the capacity to dissipate metabolically produced heat depends for the most part on the subject's ability to form and vaporize sweat (Costill, D. L. and K. E. Sparks [1973] "Rapid fluid replacement following thermal dehydration," J. Appl. Physiol. 34(3):299-303; Greenleaf, J. E. [1979] "Hyperthermia and exercise," Int. Rev. Physiol. 20:157-208).
During exercise in a hot environment, serious deficits in effective circulating volume may occur. Muscular work, independent of environment, results in massive shunting of blood to skeletal muscle, along with a substantial loss of plasma volume into the working muscle. Moreover, effective circulating volume is also diminished by losses of sweat (Knochel [1980] supra). The deficit in intravascular volume impedes the delivery of heated blood to the periphery for evaporative cooling. Thus, in the dehydrated exercising subject, there is a progressive increase in the core body temperature as sweat losses accumulate. Indeed, salt and water depletion are important predisposing factors to the development of heat-related illnesses.
Exercise is characterized by a marked increase in glucose utilization. The exercising muscle has a greatly increased need for energy. Some of the glucose needed for energy comes from liver glycogen stores. With prolonged exercise, liver glycogen stores are depleted and the rate of glucose production fails to keep pace with glucose utilization, resulting in a fall in the blood glucose concentration. The development of frank hypoglycemia has been described in marathon runners (Felig, P., A. Cherif, A. Minagawa et al. [1982] "Hypoglycemia during prolonged exercise in normal men," N. Engl. J. Med. 306(15):895-900).
Notable among the many physiological responses to physical exertion are increased body temperature, perspiration and pulse rate, a decrease in the blood volume, and biochemical changes associated with the metabolism of compounds to produce energy.
One metabolic change which is associated with continued physical exertion is a shifting of the type of compound used as the primary energy source. In the absence of physical exertion, the metabolism of fat is a primary energy source for the body. During times of exertion, carbohydrates are increasingly used as a source of readily available energy. The body continues to utilize carbohydrates as a major source of energy during prolonged periods of exercise.
If, however, the exercise is particularly strenuous or long in duration, the supply of readily available carbohydrates may become depleted and the body is forced to utilize another source of energy. The metabolism of proteins fills the energy void caused by the depletion of carbohydrates. Unfortunately, the metabolism of protein is not an efficient source of energy for the exercising individual. Protein metabolism results in the utilization of amino acids. This amino acid utilization can result in the depletion of essential amino acids in the plasma. The loss of amino acids can detrimentally affect the person or animal in many ways. One detrimental effect of the depletion of amino acids is a reduction on the body's ability to repair tissue which is damaged in the course of the strenuous exercise.
Attempts have been made to counteract the adverse effects of strenuous exertion. For example, the consumption of water helps to maintain body temperature and blood volume. This technique has met with very limited success, however. Also, products have been developed recently which combine sugar and electrolytes with water. One well known example of this type of product is GATORADE.RTM. which contains 21 millequivalents per liter (21 meq/l) of sodium, 2 meq/l potassium, and 6% sucrose. The GATORADE.RTM. composition is described in British Patent No. 1,252,781, which issued to Bradley et al. Other such compositions are known and are described, for example, in U.S. Pat. Nos. 4,042,684 and 4,322,407.
It is well known that glycerol (glycerin) can be ingested safely. Limited clinical studies have suggested that glycerol, in solution with water, may be used to induce hyperhydration (Riedesel, M. L., D. Y. Allen, G. T. Peake, and K. Al-Quattan [1987] "Hyperhydration with glycerol solutions," J. Appl. Physiol. 63(6):2262-2268). The work of the Riedesel group which was described in the 1987 publication has also been described, in part, in other places. In a 1985 abstract and in a 1987 abstract, Riedesel and coworkers reported that ingestion of an approximately 23% glycerol solution in saline resulted in overhydration of the subjects. The two abstracts had conflicting results regarding whether sweat rates were increased (Lyons et al. [1987] "Physiological Costs of Exercise Following Hyperhydration with Glycerol," Temperature Regulation I (35-40), p. 323 [abstract]; Allen et al. [1985] Environ. Physiol. II [3713-3720] p. 1046 [abstract]). In a 1987 report to the Air Force, Riedesel reported overhydration of rats which were fed glycerol (Riedesel [1987] "Oral Glycerol Solutions as a Deterrent to Dehydration During Heat Exposure," Department of the Air Force Report, AD-A118746). In a 1988 abstract Riedesel et al. again reported hyperhydration and decreased urine output after glycerol ingestion (Meuli et al. [ 1988] Exercise II [1309-1314] p. a521).
Other researchers have also examined the effects of glycerol ingestion. Maughan and Gleeson found that ingestion of large amounts of glycerol after a 36 hour fast did not significantly improve performance of exercising subjects (Maughan et al. [1988] The Eur. J. Appl. Physiol. 57:570-576). In fact, for one of the control groups, exercise duration after glycerol ingestion was lower than after water ingestion alone. This 1988 article confirms earlier work by Gleeson and Maughan which found that ingestion of large amounts of glycerol did not enhance exercise performance (Gleeson et al. [1986] The Eur. J. Appl. Physiol. 55:645-653).
Researchers at Washington University School of Medicine have also examined the effects of glycerol ingestion. In 1981 the Washington University group reported that glycerol-fed rats had increased endurance, apparently because glycerol protected against hypoglycemia (Terblanche et al. [1981] J. Appl. Physiol 50(1):94-101). Significantly, however, two years later the Washington group found that glycerol did not increase endurance in man when administered according to their protocol (Miller et al. [1983] Medicine and Science in Sports and Exercise 15(3):237-242). These published reports on the effects of glycerol have revealed that ingestion of large amounts of glycerol can result in decreased urine output and hyperhydration. Several studies have specifically looked at the effect of glycerol on endurance, and each of these studies has found that glycerol in large doses does not appear to increase endurance in man.
Much of the previous research has focused on the ability of glycerol to cause water retention. However, water retention alone has little or no correlation with enhanced endurance or physiological performance. In order to have a beneficial effect on endurance and performance, the water must be appropriately allocated throughout the body. It is not enough to simply reduce urine output. Water must be available for sweating, cells cannot be dehydrated, and plasma volume must be maintained. Only if these physiological objectives are met can endurance and performance be enhanced. This enhancement of the physiological response to exercise and heat can be largely attributed to efficient cooling of the body.
Osmotic pressure is primarily responsible for the direction and rate of movement of water across membranes in the body. The general concepts of osmosis and osmotic pressure are very well known chemical phenomena whereby water moves across a semipermeable membrane in such a way as to make its thermodynamic activity uniform across the entire system. Thus, water will move across a semipermeable membrane such that the net flow of water will be across the membrane into the fluid which initially had the highest concentration of solutes. The allocation of water between digestive organs, blood plasma, and cells depends upon the relative osmotic pressures between these sites. Although it has been established that the ingestion of massive amounts of glycerol results in the retention of water within the body, i.e., the rate of urine flow is decreased, this observation alone produces no information as to whether the body's physiological responses to heat or physical exertion have been enhanced. For example, a large concentration of glycerol in the stomach or intestine can cause water to move across the gastrointestinal membranes into the digestive tract. This might cause detrimental responses to physical exertion and heat exposure. Also, high concentrations of glycerol in the blood plasma can cause water to leave the cells and enter the plasma. Again, the resulting dehydration of the cells could have detrimental effects on the person or animal.
Studies where large amounts of glycerol have been administered in short time periods have not shown beneficial physiological effects. The researchers have observed water retention, but none have demonstrated any effect which would enhance endurance or lessen a person's discomfort. These studies do not establish any relationship between the administration of glycerol and actual physiological responses to exercise or heat exposure. Also, no studies have examined the physiological effects of glycerol or related compounds in solution with compounds other than water or saline.
Thus, the focus of glycerol research in the past has been primarily to achieve random generalized water retention. By contrast, the research which has led to the subject invention concentrated on achieving appropriate water distribution within the body. This has led to the formulation of a novel composition which greatly enhances the physiological response to physical exertion and heat exposure.
Although GATORADE.RTM. does help to combat some of the negative effects resulting from physical exertion, long distance runners and other athletes who must endure long periods of strenuous exercise still suffer the effects of decreased blood volume and a loss of energy-providing carbohydrates.
The invention described here is a novel fluid composition which surprisingly and advantageously maintains blood volume at levels well above those observed in the absence of fluids or even with GATORADE.RTM.. The novel product has the additional advantage of providing an energy source. Further, users of the product report lower levels of perceived difficulty of exercise when the novel fluid composition is used.